The present invention pertains to the field of RF radiating elements and, in particular, to dual-polarization radiating elements that can be extended via RF control networks to select any desired polarization in space and which are suitable for use in phased array antennas.
It is well known to those of ordinary skill in the art that when dual-polarization radiating elements are utilized to provide a phased array antenna, it is desirable and advantageous for the phase centers of the dual-polarization elements to coincide. In particular, it is also well known to those of ordinary skill in the art that the requirement of phase center coincidence is more important when one fabricates a phased array antenna which is responsive to wide frequency variation or bandwidth than for one of lower bandwidth.
In addition, it is well known to those of ordinary skill in the art that one type of radiating element which may operate with any one of a number of polarizations such as, without limitation, linear polarization, circular polarization, and elliptical polarization, is sometimes referred to as a "double-ridged" horn radiating element. Such a double-ridged horn radiating element has a vertical feed and an independent horizontal feed and the phase centers associated with the feeds are coincident. However, it is well known to those of ordinary skill in the art that to attain a relatively wide scan angle, say of the order of .+-.60.degree. it is generally required that the phase centers of adjacent ones of a plurality of radiating elements in an array be displaced from one another by less than one-half the wavelength. Since, the width of a horn is generally required to be larger than one-half the wavelength, and sometimes even up to the order of one wavelength, to provide efficient matching to free space over a wide frequency variation or bandwidth, it follows that while a double-ridged horn is adapted to operate with radio frequency energy of one of a variety of polarizations, such a radiating element may not be readily used in a phased array antenna having a relatively wide bandwidth and a relatively wide scan angle.
U.S. Pat. No. 3,836,976, issued on Sept. 17, 1974, discloses a closely spaced orthogonal array which attempts to solve the above-described problems associated with fabricating a phased antenna array from double-ridged horn radiating elements. In particular, this patent discloses a phased array antenna which is comprised of a plurality of vertical radiating elements and a plurality of horizontal radiating elements which are arranged in a linear array and which are affixed to a back wall which forms a ground plane for the radiating elements. However, the disclosed phased array antenna suffers several drawbacks. The first drawback of the disclosed phased array antenna is caused by the fact that all the radiating elements are notched flares which are identical and the feeds displaced from one another. As a result, the phase centers for the horizontal and vertical pair from each radiating element in the array do not coincide. As is well know to those of ordinary skill in the art, this creates a problem when the antenna is scanned in a broad band mode. The second drawback of the disclosed structure is caused by the ground plane. The ground plane causes large reflections of incident signals which can be detrimental in certain applications.
As one can readily appreciate from the above, there is a need in the art for a dual-polarization radiating element which can be used as a single radiating element or which can be combined through an RF device into a variety of phased array configurations: (1) wherein the phase center of each of its constituent radiating elements coincide to provide suitable operation in wide bandwidth, wide scan-angle, phased array antennas and (2) which does not cause large reflections of incident signals therefrom. Additionally, there is a need for such a dual-polarization radiating element which does not suffer from the mechanical cross-over problems which have, up until now, plagued the manufacture of dual-polarization radiating elements.